1. Field of the Invention
The present invention relates to dust suppression, and more particularly to suppression of dust emanation from coal by application of a dust suppressant composition to the surface of a coal pile or railcar.
2. Description of the Prior Art
Many efforts have been undertaken over many years to avoid or to alleviate the problem of dust emanation from handling, conveying, transporting and even storing masses of coal. As used herein, the phrase “mass of coal” refers to any conglomeration of coal lumps or pieces. At one time, petroleum-based compositions were applied to coal masses to suppress dust emanation from the coal. However, with the rising prices of petroleum, more recent efforts typically have involved the use of water and chemical dust suppressants that are applied to the coal. Although the categories of dust suppressants have overlapped to some extent in that certain types of suppressants may be reformulated to be applied through a system designed for another type, conventionally a suppressant may fall into the category of a short term dust suppressant, which may be a wet suppressant or a foam suppressant, or a long term residual dust control suppressant. Short term dust suppressants function typically by coating the source and dust with water. Thus, such suppressants lose their effectiveness upon exposure to various environments and weather conditions. For example, they tend to become less effective as the water evaporates upon exposure to heat, dry air or the Sun, or as rain washes the suppressant away. Moreover, their effectiveness generally does not persist beyond one to two impact points; that is, points at which dust is generated during handling or movement of the coal or other source. Examples of wet and foam suppressants are discussed in U.S. Pat. No. 4,737,305 to Dohner, U.S. Pat. No. 4,836,945 to Kestner, U.S. Pat. No. 4,971,720 to Roe, and U.S. Pat. No. 5,409,626 to Muth, and in Surfactants and Interfacial Phenomena, 2d Edition, by Milton J. Rosen, Wiley Interscience Publications (1989), all of which are incorporated herein by reference. Conventionally, long term residual dust suppressants control dust by means of the formation of a polymer or binder film over the dust source and thereby persist even after evaporation of water in the suppressant. Such suppressant compositions typically contain a binder composition such as lignosulfonate and a polymeric dispersant. For example, U.S. Pat. No. 5,436,429 to Bennett describes such a long term dust suppressant and mentions in passing that certain sugar by-products may be used as a binder or tackifier. However, binders such as lignosulfonates and polymeric binders or dispersants are costly and may create waste disposal problems in that they add additional ingredients to the coal.
Wet suppression is based on what is called “wet technology.” This suppression can be as simple as spraying large amounts of water on the coal (or other source) as it travels along a conveyor or drops to a storage pile or transfer bin. However, although water is an effective dust suppressant, its use involves a number of drawbacks, such as adding weight to the coal or other source (which can result in higher costs for transporting the source), development of substantial vapor pressure when the coal or other source is heated or burned, and the absorption and thus wasting of substantial energy as the water vaporizes when the coal or other source is heated or burned.
As a result, surfactants or “wetters” often are added to the water to reduce the amount of the water needed for dust suppression. Conventional wetters include nonionic epoxide (e.g., ethylene oxide or propylene oxide) derived block co-polymers, alcohols of from about eight to about sixteen carbons ethoxylated with from about four to about ten moles of ethylene oxide (wherein the alcohol may be an aromatic such as alkyl phenol, preferably nonylphenol, which can be ethoxylated with, for example, ten moles ethylene oxide), and branched nonionic surfactants such as branched alpha sulfo ester salts comprising an acid chain and an alcohol chain of from about six to about twelve carbons each, and highly branched alcohol sulfate and alcohol ether sulfate detergents. Generally, the wetter is added to the water in a weight proportion of from about 0.1 to about 5 parts of the wetter to about one thousand parts of water. As this concentration, the wetter acts at the interface between the coal (or other source) and the water to increase the affinity of the coal and water, thereby decreasing the amount of water needed to soak the coal as well as decreasing the time required for the water to penetrate the coal stream.
Typically, such wet technology is employed to suppress dust generated at transfer points, areas where the coal falls freely from one point to another (free falls) such as loading points where the coal is dropped into a vessel for transportation, impact points where the coal strikes a surface, transfer points where coal drops from one conveyor to another, and storage areas. The water is applied at the point of dust generation and is applied to the air-borne dust as well as to the source of the dust. If the amount of water added to the coal is sufficiently great, the coal can be prevented from dusting significantly on impact. For such benefits, the water should be added in an amount sufficient to result in a proportion of one-quarter to one part by weight water per one hundred parts by weight wetted coal. However, because the suppressant is effective only through one or two impact points where dust could be generated, repeated applications are necessary, thereby increasing the water content of the wetted coal quickly to around one or two percent. Such suppression typically will last no more than a few hours, and even less in acidic environments.
Foam suppressants are applied to form a blanket over the coal or other source to capture and smother dust. Bubbles in the foam suppressant catch the dust particles and so the foam suppressant is effective only until the bubbles break or the layer of foam becomes discontinuous. The foam suppressant is formed by addition of a foamer to water. Conventional foamers include alpha-olefin sulfonates, alkylphenyl sulfonates with long alkyl chains (e.g., eight to sixteen carbons) such as sodium lauryl benzene sulfonate, alcohol sulfates, alcohol ether sulfates, alpha sulfo esters and mixtures of such compounds. Under standard conditions, from about four to about twenty parts by weight foamer is added to about one thousand parts by weight water. The resulting foam has about five to ten percent of the density of the water used in wet technology and so much less water is needed for a foam to provide the same dust suppression as the wet suppressant. Thus, the foam suppressant can be added to the coal in a proportion such that the wetted coal contains 0.2 to 0.5 parts by weight added water per one hundred parts by weight wetted coal. However, the foam suppressant is effective only to the first impact point, after which it no longer maintains a continuous foam blanket over the dust generating material. Thus, repeated applications are necessary and the water content of the wetted coal increases quickly to several percent.
As with wet suppressants, the foam suppressants are employed to suppress dust generated at impact or transfer points, areas where the coal falls freely from one point to another (free falls) such as loading points where the coal is dropped into a vessel for transportation, impact points where the coal strikes a surface, transfer points where coal drops from one conveyor to another, and storage areas. The foam is applied as a curtain or barrier to capture generated dust. The foam applicator nozzles are located in such a way that the remaining foam and the captured dust are deposited back onto the moving coal stream. This orientation not only prevents dust from escaping into the environment, but also places at least a partial blanket of foam onto the deposited coal, which may prevent dust generation until bubbles are broken or disrupted by another transfer point. The dust suppression effects of normal foam do not carry over from a previous application point to further impact zone.
Long term residual dust control suppressants are used to prevent generation of dust during long term storage or during transportation. Such long term residual suppressants operate by a mechanism very different from those of short term residual suppressants to which the subject invention is directed. In short term residual (wet or foam) suppression, the water eventually evaporates, rendering the suppressant ineffective in suppressing dust over a long term, such as during several days of outdoor storage. Long term residual dust suppressants remain active long after the water evaporates. They ordinarily have some film-forming or tackifying properties. For example, U.S. Pat. No. 4,801,635 to Zinkan et al. describes a long term residual dust suppressant that includes an acrylic polymer and U.S. Pat. No. 4,169,170 to Doeken describes a long term residual dust suppressant that includes an aqueous suspension of asphalt emulsion concentrates or black liquor lignin products such as lignosulfonates as a binder material. Conventionally, water is included in a long term residual suppressant typically not only to provide some dust control during handling prior to storage, but also to promote even spreading as the suppressant is applied. Long term residual dust suppressants often contain wetters or foamers as well to minimize the amount of water needed to apply the suppressant to the coal and are applied directly to the coal in what is known as a “main body treatment.”
Thus, several problems are associated with conventional wet and foam dust suppression techniques to which the subject invention is directed. For example, each technique involves addition of a substantial amount of water to the coal or other dust source, especially in view of the repeated applications of water-based suppressant necessary to control dust across several impact or transfer points. The resulting high water content of the coal is particularly undesirable in that much dust suppression is performed at fossil fuel power plants. Water added to the fuel results in a portion of the heat output of the coal to be lost to vaporization of the water and so a loss of effective energy. The vaporization of water consumes substantial amounts of heat. Therefore, the addition of such significant amounts of water is particularly troublesome. In addition, the additional water increases the weight of the coal and so increases shipping costs accordingly.
In addition, binding agents, wetting agents, lignosulfonates, polymers and volatile organic compounds (“VOC's”) are often required in the conventional dust suppressants. For example, U.S. Pat. No. 4,417,992 teaches a suppressant comprising acrylamides and acrylamide polymers. U.S. Pat. No. 4,238,536 describes a petroleum/rosin gel. Such ingredients often can be undesirable as well in that their use increases the cost of the suppressant and can raise problems in disposal and clean-up and other problems associated with the transport and burning of the coal.
U.S. Pat. No. 4,642,196 discloses a method for controlling coal dust with a suppressant based on a gelatinized starch, but the suppressant techniques disclosed therein appear to employ relatively large quantities of starch (0.05% to 10% based on pounds of starch to pounds of coal), with the concomitant disadvantages of high volume suppressant applications as discussed above, in a high concentration composition. Further, the reported composition is of high viscosity, rendering handling difficult, thereby requiring application at elevated temperatures. Moreover, application of the suppressant in the noted patent apparently not as a blanket barrier over the mass of coal, but as a penetrating composition that permeates throughout the mass of coal for bulk treatment as opposed to blanketing the mass of coal to produce a crust thereover. U.S. Pat. No. 4,642,196 does refer to application “on top of the coal car and the top of stock piles.” However, a reading of the patent in its entirety suggests that the inventor therein teaches appropriate sites for placement of spray manifolds for application of the treatment of moving coal.
Because of the substantial disadvantages associated with the addition of such significant amounts of water, the industry has attempted to minimize the amount of water employed in wet and foam suppression techniques. Such attempts typically involve the use of systems for application of the suppressant at each dust producing site instead of a single application that would be intended endure various environmental activity for relatively long-lasting suppression despite evaporative conditions or rain or even both. Because the suppressant in the multiple application technique remains effective for only one or two transfer points and are destroyed by evaporation or rain, such techniques are expensive; they require costly installation of application equipment at several transfer points, impact points and loading or “stack-out” storage sites. In addition, impact sites where dust is generated often are not accessible to the equipment employed in conventional application systems. Thus, such techniques are undesirable.